This invention relates generally to raster scanning systems and more particularly, to a raster output scanner which utilizes a collimator assembly which can be replaced in the field at a customer location without disassembling the raster output scanner and sending it back to the manufacturer for replacement.
Typically, a laser printer utilizes a raster output scanner. Referring to FIG. 1, there is shown a tangential (fast-scan) view of the raster output scanner 10 of a printing system. The raster output 10 utilizes a laser light source 12, a collimator 14, mirrors 16 and 18, pre-polygon optics 20 and 22, mirror 24 a multi-faceted rotating polygon mirror 26 as the scanning element, post polygon optics 28, mirror 30 and a photosensitive medium (photoreceptor) 32.
The laser light source 12 sends a light beam 34 to the rotating polygon mirror 26 through the collimator 14 and the pre-polygon optics 20 and 22. Mirrors 16, 18 and 24 fold and redirect the light beam 34 prior to the scanning polygon 26 and mirror 30 folds and redirects the light beam 34 after the scanning polygon 26. Mirror 30 is slanted to redirect the light beam 34 outside of the ROS housing onto the photoreceptor 22 to scan a line S.
The collimator 14 collimates the light beam 34 and the pre-polygon optics 20 focuses the light beam 34 in the sagittal or cross-scan plane onto the rotating polygon mirror 26. However, since this system is an overfilled system, the light beam stays collimated in the tangential plane while striking the polygon mirror 26. The facets 36 of the rotating polygon mirror 26 reflect the light beam 34 and also cause the reflected light beam 34 to revolve about an axis near the reflection point of the facet 36. The reflected light beam 34 is utilized through the post polygon optics 28 to scan the photoreceptor 32.
Typically, all the above optical elements except the laser light source 12 and the collimator 14 are placed in a Raster Output Scanner (ROS) housing 38. The laser light source 12 and the collimator 14 are placed in a collimator assembly 40 which is mounted onto the ROS housing 38.
Referring to FIG. 2, there is shown an isometric view of the collimator assembly 40 and a portion of the ROS housing 38 of FIG. 1. Referring to both FIGS. 1 and 2, ROS housings 38 is usually made of plastic or metal and has an opening 42 for receiving a light beam from the laser light source 12. The collimator assembly 40 holds the laser diode 12 and the collimator 14. The base 46 of the collimator assembly 40 is mounted on wall 44 of the housing 38 in such a manner that the axis 48 of the collimator assembly coincides with optical path 35. The optical path 35 is the optical axis of the optical elements within the ROS housing 38. The laser diode 12 emits a light beam 34 and collimator 14 collimates and sends the light beam 34 into the ROS housing 38 through the opening 42. Within the ROS housing 38, the light beam 34 travels along the optical path 35.
During manufacturing, after the collimator assembly 40 is mounted on the ROS housing 38, the position of the light beam 34 from the laser diode has to be adjusted to overlap the optical path 35 and the intensity of the light beam 34 has to be adjusted to match a required discharge level of the photoreceptor used in that specific printer. Adjusting the position and the intensity of the light beam 34 are very critical in the print quality. For example, if the light beam 34 does not travel on the optical path 35, the light beam striking the photoreceptor might be out of focus which causes the print to be blurred. In addition, if the intensity level of the light beam happens to be over or under the required level, the print will be darker or blank respectively.
The above adjustments are done based on the location and the characteristics of the laser diode. The pointing of a laser diode with respect to the optical path 35 depends on the mounting of the collimator assembly to the ROS housing. Furthermore, characteristics of each individual laser diode is different from characteristics of other laser diodes. Therefore, in order to adjust the intensity of a laser diode, the laser driving current has to be adjusted. As a result, if a laser diode of a printer needs to be replaced, the whole ROS housing is used to readjust a new replacement collimator assembly.
Therefore, in order to replace a laser diode, the ROS housing, including the collimator assembly, has to be dismounted from the printer and sent back to the manufacturing. Since the ROS housing holds expensive optical elements, it is desirable not to transfer it back to manufacturing to prevent any damage to the optical elements. Furthermore, transferring the ROS housing to the manufacturing for repair or replacement can be very costly to the user in terms of loss of productivity. Therefore, it is advantageous to replace the collimator assembly in the field instead of sending the ROS housing back to the manufacturer.
It is an object of this invention to design a raster output scanner with a field replacable laser diode.